The present invention relates to a cylindrical magnet apparatus, and more in particular to a cylindrical permanent magnet or an electromagnet apparatus suitable for use with a nuclear magnetic resonance (NMR) imaging apparatus (hereinafter called "MRI").
A magnetic field generating apparatus (hereinafter called "the magnet") used with an NMR imaging apparatus has a magnetic field generating space accommodating therein a compensating coil for improving the magnetic field homogeneity, a gradient coil (GC) for generating a gradient magnetic field in X, Y and Z directions, an NMR irradiation coil and a detection coil in which a human body or a human head is inserted, and a part of table for holding the human body.
FIGS. 1A and 1B show an embodiment of a generally-used conventional permanent magnet apparatus of so called "WE-type", in which FIG. 1A specifically shows a side view along the plane Y-Z, and FIG. 1B a sectional view along X-Y plane taken in line IB--IB in FIG. 1A. Numeral 2 designates permanent magnet members constituting a magnetic field source, numeral 3 pole pieces for making uniform the magnetic fluxes generated from the permanent magnet members 2, and numeral 4 a magnetic field generating space into which the magnetic fluxes thus made uniform flow. A magnetic yoke 1 made of a ferromagnetic material (generally iron) is so constructed as to confine the magnetic fluxes in the magnet system.
The gap size between the oppositely-placed pole pieces 3 is designated as g, and the surface diameter of the pole pieces 3 as D. FIGS. 2A and 2B show an embodiment of a conventional air-core four-coil resistive electromagnet (hereinafter called "RM") most widely used for routine MRI, in which FIG. 2A is a side view along the Y-Z plane, and FIG. 2B a sectional view of the X-Y plane as viewed in line IIB--IIB in FIG. 2A. In FIGS. 2A and 2B, numerals 5, 6 designate electromagnet coils wound with an anodized aluminum strip respectively. Numeral 8 designates an opening of the electromagnet coil, and numeral 7 is a space in which a magnetic field is generated when current flows through the electromagnet coils 5, 6.
In the WE-type permanent magnetic apparatus shown in the drawing, the gap size g of the magnetic field space 4 is at least 70 cm, and RM shown in FIG. 2 requires a magnetic field-generating space 7 of 90 cm in minimum inner diameter and a magnet opening of 60 cm in minimum inner diameter for accommodating the human body. The magnetic field-generating space, is required not only to generate a magnetic field but also to secure a homogeneous magnetic field of at least 150 ppm (parts per million) in 40 cm dsv (diameter of spherical volume) and 50 ppm in 30 cm dsv. This makes the magnet now used with MRI extremely large in size. Typical examples of the magnets now available on the market are as follows:
(i) In generating a magnetic field of 0.15 T (tesla) by RM the above-mentioned homogeneity of magnetic field is obtained when the magnet opening is 80 cm in diameter, and the inner diameter of the magnetic field-generating space is 100 cm in diameter, requiring the weight of about three tons, electric power of about 50 KVA, AC and the cooling water of about 50 l/min.
(ii) In generating a given magnetic field of 0.5 to 2.0 T (tesla) by a super conductive magnet (SCM), the inner diameter of about 100 cm, the weight of about 4.5 tons, the quantity of liquid helium (He) of about 800 l and the quantity of liquid nitrogen (N.sub.2) of about 500 l are required, while consuming great quantities of refrigerants including liquid He at the rate of 0.5 l/hr and liquid N.sub.2 at the rate of 2 l/hr.
(iii) In general, the permanent magnet apparatus, does not necessitate electric power, cooling water or refrigerant, while the WE-type permanent magnet apparatus requires the weight of about 10 tons for the magnetic field of 0.05 T (tesla) and about 100 tons for 0.3 T (tesla).
MRI, which is used for clinical diagnosis in hospitals, is desirably a safe and stable system including a simple equipment provided with a power supply, and cooling water, refrigerant and air-conditioning systems, easy in maintenance and control, and low in maintenance, operation and other costs. For routine MRI, therefore, the permanent magnet apparatus is the best choice as a magnet.
As described above, the WE-type permanent magnet apparatus, if built by the prior art, is excessively heavy making transportation and carriage into a building and installation therein very troublesome. The weight of about ten tons do not provide a large problem, but the associated magnetic field of 0.05 T (tesla) fails to meet the current and future requirements. Even for routine MRI, the magnetic field of at least 0.1 to 0.15 T (tesla) is necessary. If this intensity of magnetic field is to be obtained by the prior art, the weight of as much as 30 to 50 tons will be required.
The reasons for the requirement of this great weight will be discussed below.
In MRI, a homogeneous magnetic field is required over a wide area of the magnetic field-generating space as mentioned above. In the conventional WE-type permanent magnet apparatus, the homogeneity of magnetic field (.DELTA.H.sub.0 /H.sub.0) is given as EQU .DELTA.H.sub.0 /H.sub.0 .about.10.sup.-(D/g+1) ( 1)
where
.DELTA.H.sub.0 : Magnetic field irregularities PA1 H.sub.0 : Central magnetic field PA1 g: Gap size PA1 D: Diameter of pole piece (in disc form)
In order to improve the uniformity of magnetic field, therefore, it is necessary to reduce the gap size g and to increase the diameter D. For NMR imaging, however, the gap size g of at least 70 cm is required, and therefore, if .DELTA.H.sub.0 /H.sub.0 .about.10.sup.-5 is required for 30 cm dsv, the diameter D may be 280 cm (=70 cm.times.4). The diameter D of at least 200 cm is required even with various auxiliary means for correcting the homogeneity. As obvious from FIG. 1, with the increase in diameter D, the permanent magnet members 2, increase in size, thereby increasing the magnetic yokes 1, very greatly. As a result, the weight of approximately 100 tons is involved for 3000 gauss.